1. Field of the Invention
The present invention relates to a spin valve magnetoresistive sensor or transducer and a spin valve magnetoresistive head using the sensor.
2. Description of the Related Art
In association with a reduction in size and an increase in recording density of a magnetic disk drive in recent years, the flying height of a head slider has become smaller and it has been desired to realize contact recording/reproduction such that the head slider flies a very small height above a recording medium or comes into contact with the recording medium. Further, a conventional magnetic induction head has a disadvantage such that its reproduction output decreases with a decrease in peripheral speed of a magnetic disk as the recording medium (relative speed between the head and the medium) caused by a reduction in diameter of the magnetic disk. To cope with this disadvantage, there has recently extensively been developed a magnetoresistive head (MR head) whose reproduction output does not depend on the peripheral speed and is capable of obtaining a large output even at a low peripheral speed. Such a magnetoresistive head is now a dominating magnetic head. Further, a magnetic head utilizing a giant magnetoresistive (GMR) effect is also commercially available at present.
With higher-density recording in a magnetic disk drive, a recording area of one bit decreases and a magnetic field generated from the medium accordingly becomes smaller. The recording density of a magnetic disk drive currently on the market is about 10 Gbit/in2, and it is rising at an annual rate of about 200%. It is therefore desired to develop a magnetoresistive sensor and a magnetoresistive head which can support a minute magnetic field range and can sense a change in small external magnetic field.
At present, a spin valve magnetoresistive sensor utilizing a spin valve GMR effect is widely used in a magnetic head. In such a magnetoresistive sensor having a spin valve structure, a magnetization direction in a free ferromagnetic layer (free layer) is changed by a signal magnetic field from a recording medium, so that a relative angle of this magnetization direction to a magnetization direction in a pinned ferromagnetic-layer (pinned layer) is changed, causing a change in resistance of the magnetoresistive sensor. In the case of using this magnetoresistive sensor in a magnetic head, the magnetization direction in the pinned layer is fixed to a direction along the height of a magnetoresistive element, and the magnetization direction in the free layer in the condition where no external magnetic field is applied is generally designed to a direction along the width of the magnetoresistive element, which direction is perpendicular to the pinned layer. Accordingly, the resistance of the magnetoresistive sensor can be linearly increased or decreased according to whether the direction of the signal magnetic field from the magnetic recording medium is parallel or antiparallel to the magnetization direction of the pinned layer. Such a linear, resistance change facilitates signal processing in the magnetic disk drive.
In the conventional magnetoresistive sensor, a sense current is passed in a direction parallel to the film surface of the magnetoresistive element to read a resistance change according to an external magnetic field. In such a case of a CIP (Current In the Plane) structure that a current is passed in a direction parallel to the GMR film surface, the output from the sensor decreases with a decrease in sense region defined by a pair of electrode terminals. Further, in the spin valve magnetoresistive sensor having the CIP structure, insulating films are required between the GMR film and an upper magnetic shield and between the GMR film and a lower magnetic field. That is, the distance between the upper and lower magnetic shields is equal to the sum of the thickness of the GMR film and a value twice the thickness of each insulating film. At present, the thickness of the insulating film is about 20 nm at the minimum. Accordingly, the distance between the upper and lower magnetic shields becomes equal to the sum of the thickness of the GMR film and about 40 nm. However, with this distance, it is difficult to support a reduction in length of a recording bit on the recording medium, and the current CIP spin valve magnetoresistive sensor cannot meet the requirement that the distance between the magnetic shields is to be reduced to 40 nm or less.
In these circumstances, it is considered that a magnetic head having a CIP structure utilizing a spin valve GMR effect can support a recording density of 20 to 40 Gbit/in2 at the maximum. Even by applying specular scattering as a latest technique, the maximum recording density is considered to be 60 Gbit/in2. As mentioned above, the increase in recording density of a magnetic disk drive is rapid, and it is expected that a recording density of 80 Gbit/in2 will be desired by year 2002. When the recording density becomes 80 Gbit/in2 or higher, it is very difficult to support such a high recording density even by using a CIP spin valve GMR magnetic head to which the latest specular scattering is applied, from the viewpoints of output and the distance between the magnetic shields.
As a post spin valve GMR intended to cope with the above problem, there have been proposed a tunnel MR (TMR) and a multilayer CPP (Current Perpendicular to the Plane) structure. The TMR has a structure that a thin insulating layer is sandwiched between two ferromagnetic layers. The amount of a tunnel current passing across the insulating layer is changed according to the magnetization directions in the two ferromagnetic layers. The TMR shows a very large resistance change and has a good sensitivity, so that it is expected as a promising post spin valve GMR. On the other hand, the multilayer CPP structure has an effect that when a current is passed in a direction perpendicular to the film surface of a GMR film (in a direction including at least a vertical component), the resistance change in the GMR film is almost doubled at room temperature, thereby improving the output. Further, in the case of the CPP structure, the output increases with a decrease in sectional area of a portion of the GMR film where a sense current is passed. This feature of the CPP structure is a large advantage over the CIP structure.
The TMR is also considered to be a kind of CPP structure, because a current is passed across the insulating layer from one of the ferromagnetic layers to the other ferromagnetic layer. Therefore, the TMR also has the above advantage. Thus, the TMR and the multilayer CPP structure are promising. However, these structures have not yet been put to practical use, and have some problems.
For example, the multilayer CPP structure has the following problems.
(1) An element (device) fabrication process is complicated to require a high accuracy.
In fabricating a device having the multilayer CPP structure, a process including film formation, resist formation, ion milling or reactive ion etching (RIE), and resist removal must be repeated at least three times, and a very high positioning accuracy is required in the step of resist formation. Further, in the step of ion milling or reactive ion etching, a technique of stopping the milling or etching in the middle of the thickness of a metal layer before reaching a substrate surface is also required.
(2) The evaluation of characteristics is difficult unless the size of a CPP portion is about 1 μm or submicrons.
When the size of a CPP portion becomes about 3 μm or more, a voltage to a sense current is measured as a negative value by the influence of current distribution. As a result, in the case that the size of the CPP portion is about 3 μm, the MR ratio becomes a very large value. Accordingly, a conventional evaluation standard cannot be applied.
(3) The characteristics are largely dependent on whether or not the device fabrication process is well performed.
While this tendency also applies to a conventional GMR having a CIP structure, it is remarkable in the case of the CPP structure. That is, the characteristics of the GMR largely change according to a sectional shape or a condition of generation of burrs in processing a GMR film or an insulating film. Accordingly, it is difficult to determine the cause of generation of defectives.
(4) Hysteresis is present, magnetic domain-control is difficult, and the thickness of a portion for sensing an external magnetic field is large.
Magnetic layers in the multilayer CPP structure are magnetically connected with each other to cause the presence of hysteresis. Further, the number of the magnetic layers is large and it is therefore difficult to perform magnetic domain control of each magnetic layer. Further, all the magnetic layers basically sense an external magnetic field to change the magnetization direction in each magnetic layer, so that the thickness of a portion for sensing an external magnetic field is large.
The above problems (1) to (3) can be solved by improving the element structure and the processing accuracy, for example. However, the problem (4) is a fundamental problem of the multilayer GMR, and there are no specific measures for solving this problem at present.
It is expected that the GMR element will be increasingly minute in structure with a further increase in recording density and that high-yield production of GMR elements will be difficult in the conventional process of directly polishing each GMR element in its vertical direction, from the viewpoint of its processing accuracy. In this respect, there is also known a-flux guide type GMR head which can eliminate the need for direct polishing of the GMR element in its vertical direction. In the case of using such a flux guide type GMR head as a conventional CIP structure, the flux guide and the GMR element must be insulated to prevent a current from separately flowing to the flux guide.
Accordingly, the flux guide and the GMR element must be sufficiently separated from each other, so that a magnetic field from the medium cannot be sufficiently transferred from the flux guide to the GMR element, causing a reduction in reproduction output. Further, when the GMR element portion is processed minutely as in the conventional CPP structure, the region where magnetization is not moved by a demagnetizing field on the end surface becomes large, causing a reduction in reproduction sensitivity.
The GMR head has a problem such that when the GMR film does not become a single magnetic domain, Barkhausen noise is generated to cause large variations in reproduction output. To cope with this problem, a magnetic domain control film for controlling a magnetic domain in the GMR film is provided. As the magnetic domain control film, a hard magnetic film formed from a high-coercivity film is used to control the magnetization direction in the free layer to a direction along the width of the element by a bias magnetic field generated from the hard magnetic film.
The intensity and distribution of the magnetic field generated from the hard magnetic film is strongly dependent on the shape of the hard magnetic film. The shape of the hard magnetic film usually formed by a lift-off process or the like is influenced by the process accuracy, and it is therefore difficult to obtain a stable bias magnetic field. In the case that the hard magnetic film has such a shape as to ride over the element, the bias magnetic field is partially applied in a direction opposite to a desired magnetization control direction, thus inducing the generation of Barkhausen noise or the like. As a result, it is considered that the element characteristics are degraded.
To cope with this problem, there is provided an exchange bonding type magnetic domain control method including the steps of laminating antiferromagnetic layers as bias magnetic field applying layers on the opposite ends of the free layer and controlling the magnetization direction in the free layer by utilizing the exchange bonding between the free layer and each antiferromagnetic layer. According to this method, the exchange bonding between the magnetic layers is utilized, so that it is possible to obtain a stabler bias effect than that by the above control method using the magnetic field from the hard magnetic film. In the case of forming such an exchange bonding type magnetic domain control element, a sufficient bias magnetic field can be obtained by continuously forming the free layer and the antiferromagnetic layer as a bias magnetic field applying layer.
However, the bias magnetic field at a magnetic field sensing portion of the free layer must be eliminated to improve the element sensitivity. In the case that the antiferromagnetic layer is formed of a less durable material such as FeMn, the antiferromagnetic layer is made to react with oxygen to form an oxide of constitutive elements of the antiferromagnetic layer, thereby eliminating the bias magnetic field at the magnetic field sensing portion. However, a more durable material such as NiMn, PtMn, PdPtMn, or IrMn than FeMn is now used as the material of the antiferromagnetic layer from the viewpoint of reliability of the magnetic head. As a result, the resistance of the antiferromagnetic layer against oxidation is improved to cause a problem that the process of eliminating the bias magnetic-field from the magnetic field sensing portion becomes difficult.